How to create a menu item as an addon that is displayed in the left side pane
Open Text Editor then choose Templates->Python you will find templates for simple menus and panels (Ui Menu, Ui Simple Menu, Ui Panel, Ui Simple Panel), run them and see how it goes, they should give you a very good understanding of what you can do.



Running the UI Simple Panel will show it under the object properties as shown below, the location of the panel is specified using these three class parameters:

bl_space_type = 'PROPERTIES'
bl_region_type = 'WINDOW'
bl_context = "object"




To put in the left side you change the values as shown below: